FIG. 1 shows the diagram of a prior art distributed access point deployment, such as a sports stadium. Many wireless users may congregate in the stadium, and enhance their sport viewing experience using portable wireless devices which include a WLAN capability. In the prior art, a plurality of access points AP1 102, AP2 104, AP3 106, AP4 108, AP5 110, AP6 112, AP7 114, AP8 116 are placed around the perimeter of the stadium, or in any manner which provides adequate WLAN coverage over the stadium. Each of the access points (AP) advertises a BSSID (Basic Service Set Identifier), which is typically the MAC address of the access point. FIG. 2 shows the wireless links 202 and 210 between access points AP1 and AP5, respectively, to station STA1 118, all of which operate together on the using shared media access on channel 1, where each channel represents a 20 Mhz band of subcarrier frequencies to prevent interference with other channels. There may be many stations such as STA1 118, which associated with nearby AP1 102 on the same channel, and AP5 110 operates on the same channel and can be a source of remote interference, as it is part of a shared media channel which is not in direct communication with STA 1 118. Although a small number of frequency channels may be available for all of the stations of the WLAN, it is desired to separate as much as possible each of the access points operating on a particular frequency from other access points operating on the same channel, as shown for channel 1 AP1 102 and AP5 110. A separate set of stations and access points may operate independently on channel 6, shown as STA2 120 and STA 3 122 near AP 8 116, and STA4 124 and STA5 126 which are near AP4 108. Other access points AP2 104 and AP6 112 are also operative on channel 6. In the example of a stadium, where multipath losses are low, one issue that arises is that the access points which share the same channel of operation such as CH6, shown in FIGS. 1 and 2, interact in a manner which reduces the overall bandwidth even though they are physically separated and the respective BSSIDs are associated with different stations.
WLAN communications systems which operate according to IEEE standard 802.11b or 802.11g operate with each station (STA) associated with a particular access point (AP), such that a plurality of stations 120 and 122 may associate with AP8 operative in CH6, and a different plurality of stations such as STA4 124 and STA5 126 may be associated with an access point AP4 which is also operative in CH6, and yet another plurality of stations (not shown) may be associated with access points AP2 and AP6. Under the IEEE 802.11b and 802.11g WLAN standards, the simultaneous transmission by access points and stations is known as a collision, and the AP and STA will re-transmit the packet when the intended recipient of the corrupted packet fails to acknowledge receipt by detecting the missing sequence number of the corrupted packet in the received packet stream. The transmitter will reduce the likelihood of collision through use of the detection of a clear channel assessment (CCA) signal. For the indoor WLAN environment with multi-path reflection, it is desired to operate in the manner, as a station may be associated with an access point through a multi-path reflection environment which produces a weak signal at the associated station or access point.
FIG. 3 illustrates one of the throughput problems associated with having several stations and access points within reception range of each other, such as in a low multi-path reflection high user density environment such as the open stadium shown in FIGS. 1 and 2. In an actual system, the BSSID would be the 48 bit MAC address of the access point, however, for clarity in understanding the invention, each access point of the present example advertises a BSSID in the form of the access point name provided as an underscore suffix. For example, the BSSID associated with AP4 is shown as BSSID_AP4 and the BSSID associated with AP8 is shown as BSSID_AP8. Each station utilizes the well-known 802.11 association protocol to “join” with a particular BSSID. The problem is that although the individual stations have joined a particular BSSID, the channel media is still shared as far as packet acquisition (detection of a packet) and packet transmission (holdoff from transmit when another station or AP is using the shared media) are concerned. FIG. 3 shows an example of this from the perspective of STA2 120, which is associated with BSSID_AP8, and on the same shared media channel 6 as “other” BSSID_AP4. “Other BSSID” 302 indicates packet 326 being transmitted by BSSID_AP8, during which time the “clear channel assessment” CCA 308 is unasserted starting at time 314. The local station STA2 120 has a transmit request 310 with associated packet ready to transmit at time 316, however as shared media, the CCA 308 signal unassertion causes the station to wait until indication of clear channel at time 317, after which time data 322 and associated packet 328 is transmitted. By comparison, at time 320 when transmit request 310 is asserted, the data 324 is immediately sent as packet 330. A similar problem occurs during packet acquisition, as shown in FIG. 4. From the perspective of STA2 120, a packet from “other” BSSID_AP4 arrives, shown in plot 402 as packet 404, and is subject to packet acquisition process by the baseband processor, which performs a preamble detect and acquisition 414, header recovery 416 and begins demodulating the packet payload 416. In this case, the BSSID which will be recovered will be undesired BSSID_AP4 of the “other” station. During this interval, a packet for “our” BSSID_AP8 arrives 406 as shown 408, which has stronger signal level as shown in RSSI 410 waveform, and the coincident stronger signal only serves to corrupt 418 the packet 404 that STA2 had started to acquire.
Therefore, a problem occurs in high density user environments where a large number of stations share a channel number and a plurality of BSSIDs are present, where local station transmission is deferred while awaiting a remote sender on a different BSSID to complete, and reception of a low signal level packet from a remote BSSID interferes with reception of a high signal level packet from a nearby BSSID.
It is desired to provide a mechanism to reduce interference from remote BSSIDs in transmission and reception of packets from a local station, and thereby increase throughput in a high density station environment with multiple access points sharing a particular wireless channel.